Not difficult at all. Remove the handguards (slip ring back so you can lift top off first, then bottom). Use Roll pin punch to push out gas tube roll pin located at the front sight base. Slide out gas tube (be sure not to "kink" it). Put barrel in barrel blocks (if you don't have a set, you can make it out of oak - take calipers to the bbl and measure and find a drill that fits. Drill & cut wood in half on bandsaw) then use wrench to loosen barrel nut. Off she comes. Viola!

Assembly is in reverse order. You should headspace it when you're done.

Headspace isn't an issue, because it's set when the barrel is put into the barrel extension at the factory. If you buy a new (or near new) barrel from a reputable manufacturer, you should have no headspace issues.

Clear the weapon and remove the upper from the lower receiver. Remove the bolt and carrier from the upper receiver. Remove the handguards and place your upper receiver in the vise blocks (or barrel, whichever you prefer, I like to use the barrel vise blocks, even though they leave aluminum on the barrel that you later have to clean off, you are less likely to damage the upper when torquing the barrel the final time.), drift out the gas tube roll pin in the front sight base. Now remove the gas tube by twisting and pulling it to the rear. Once it clears the front sight base, carefully pull it forwar, past the front sight base and out of the upper receiver.

Now put the barrel wrench on the barrel nut and loosen the barrel nut. Pull the barrel out of the upper receiver. Use the retaining ring pliers to remove the retaining ring from the back of the barrel nut. Now you can remove the spring and slip ring for use on your new barrel assembly. Remove the old barrel assembly from the vise and insert the new one. Put the slip ring and spring on the new barrel nut and using the retraining ring pliers, re-install the retaining ring into the groove in the back of the barrel nut.

Clean the threads on the new barrel nut and apply some of the molybdenum disulfide grease to them. (the grease acts as an anti-seize compound)

Wipe the threads on the upper receiver clean and make certain there are no burrs. Apply some of the molybdenum disulfide grease to them. Align the upper receiver assembly using the slot in the upper receiver and the alignment pin on the barrel extension. The alignment pin should fit in the slot almost perfectly with little to no rotational play present. Tighten the barrel nut to the upper receiver. Tighten to 30 ft-pounds. Loosen and repeat. torque operation, then loosen and repeat again. The three times torquing procedure provides for better thread fit and prevents the barrel from loosening. Now tighten the barrel nut a little more until you can align the notches in it with the hole in the upper receiver for the gas tube. Caution; Do not tighten to more the 80 ft pounds to align the notch and hole.

Use an old gas tube or a number 15 (0.180 inch) twist drill to align the parts of the barrel nut slip ring assembly with the hole in the upper for the gas tube. Insert the gas tube into the upper then into the front sight base. Drive a new gas tube roll pin into the front sigt base and you are finished.

If you are really worried about headspace use a USGI field gage and check it. There is no adjustment, so if you have excessive headspace you can either change the bolt or the barrel. In practice, I have never known headpsace to be a problem as long as you started with good parts from a reputable manufacturer.